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Since milk is mostly water, its boiling point will be close to that of water, which is around 100 degrees C (the exact number depends on atmospheric pressure). In the same way, the exact boiling point of a cup of milk will depend on what type of milk it is and what altitude it's at. Mixing salt or sugar (or other solutes) with water will raise its boiling point slightly, and a good guess for milk is that the boiling point will be a fraction of a degree C higher than that of pure water at the same atmospheric pressure. (I looked over the sodium, potassium, calcium, and lactose concentrations for the milk in my fridge. Allowing also for some negative ions to go with the positive ones, I get that the total solute concentration should raise the boiling point about 0.2°C above that of water. For further discussion on this topic, see below. /mw)

You may be interested to know that there is a special process in which milk is brought to a temperature just below its boiling point, then cooled down again. This is referred to as "scalding" and is commonly used for making yogurt, cheeses, ice creams, and other products.
(That's not as tricky as it might sound. If the milk is in a container with plain water around it, a double-boiler sort of set-up, the plain water boiling will keep the temperature from going above the boiling point of water, which is just below the boiling point of milk./mw)

To find out the exact boiling point of a glass of milk, try boiling it yourself. Measure the temperature of the milk with a thermometer just as it reaches a boil. (After the boiling starts, you will be able to heat the milk even further before it boils away, unlike pure water. The reason is that after some of the milk's water has boiled off, the remaining liquid has a higher concentration of salt, sugar, etc. left behind. That raises its boiling point further. So really the 'boiling point' is just the temperature where the boiling begins. To boil off enough water to leave only a more or less solid residue requires noticeably higher temperatures./mw ) But you should be aware that the milk probably won't taste quite the same after boiling it, so don't boil anything that you really want to drink! ;)

-Tamara

(published on 10/22/2007)

Follow-Up #1: milk boiling and fat

Q:

How does fat in milk affect the boiling point?- Megan (age 15)Myrtle Point HS, Oregon

A:

Megan- Iíll make an educated guess. In general, things dissolved in water (and milk is mostly water) will raise the boiling point. Roughly speaking, the greater the density (number per volume) of molecules dissolved, the greater the increase in the boiling point. So milkís boiling point is a little above that of water, because there are sugars, salts, and proteins dissolved in it.

My guess is that the fat has very little effect. Thatís because it isnít in solution as individual molecules but rather as little globs of molecules. These should have less effect than the same amount of stuff in solution as individual molecules. Since fat is only maybe 1/6 of the non-water ingredients of whole milk, and itís present as many-molecule globs, it should have much less effect than, say, the sugar. Thereís more sugar by weight, and itís in solution as smallish individual molecules.

You could try testing this, but I bet it would be hard to see an effect of fat without a very careful experiment.

Mike W.

(published on 10/22/2007)

Follow-Up #2: solute elevation of boiling point

Q:

In answer to Meganís question on milk fat and boiling point, you made the following remark.
Megan (age 15)
Myrtle Point HS
Oregon
A: Megan- .....Roughly speaking, the greater the density (number per volume) of molecules dissolved, the greater the increase in the boiling point. ...
My parents and I have been completely unable to find any source, on the internet or in a book, which states this relation between density and boiling point.
My Dad, who is a pretty old guy, is certain he remembers this from his high school days, but he cannot find this said anywhere.
Can you help, please???
Thank you, in advance, for your kindness, and your interesting web page.
Erin- Erin (age 12)Whitney Young, Chicago

A:

Erin- we always like follow-up questions. It shows somebody actually read an answer.

Perhaps I should explain why there's a simple relation between the number of dissolved molecules or ions per unit volume and the boiling point elevation. This explanation may be more technical than you or many other readers want, but I can't resist trying to show that there's a logic to these rules, not just a set of sayings to remember. The description below is somewhat general, but the solvent could often be water, and the solute could be salt or sugar.

A liquid boils when it's heated to a temperature for which the net free energy doesn't change when some molecules leave the liquid and join the gas (at the particular pressure used). The free energy change involves the change in both energy and entropy (see below) of the liquid and the gas. It takes a lot of energy to pull a molecule out of the liquid, but it gains a lot of entropy by having a lot of space to run around in in the gas. (Entropy is defined as the natural logarithm of the number of states available.) The relative importance of the entropy and the energy is determined by the temperature. So there's a particular temperature (the boiling point) where the two effects balance, and both the liquid and the gas are stable.

For each solvent, those effects can be pretty hard to figure out, but fortunately all we care about here is the change when a little solute is added, because that's what changes the boiling point. Let's think of what happens when you add just a little solute, not enough for there to be much interaction between the solute molecules or ions. The solute may change the entropy and energy of the solvent, but that effect isn't sensitive to changes in the number of solvent molecules because each solute molecule is completely surrounded by solvent anyway. The key change is this: when a solvent molecule evaporates, it leaves less room for the solute molecules to rattle around in. There are fewer states available to them. Another way to say that is that they have lost some entropy. If, say, 1% of the solvent boiled away each solute molecule would have only 99% as many states available to it, so it would lose about 0.01 of entropy, in natural entropy units. The entropy loss per solute molecule doesn't depend on its molecular properties, so neither does the effect on the boiling point!

mike w

This simple everyday question has some interesting physics in it, and I've re-thought the answer a few times. The question is whether for small non-volatile solute concentrations the boiling point elevation (or equivalently, the vapor pressure reduction) depends only on the concentration of solute molecules, not on any of their properties. I'm sure the argument is in physical chemistry textbooks, but these days it's easy to get lazy and try to get all info from the Web or by thinking.

At first I wrote yes, then altered the answer to say sort-of. After some thought, the answer is yes for any non-ionic solute. I'll post an update when I'm more sure about ionic solutes.

Anyway, here's the argument for non-ionic solutes. These have no long-range interactions with each other or the solvent. So you can break up the free energy into four parts:

1. from pure solvent regions.2. from little balls of solvent each containing one solute molecule3. from solute-solute interactions.4. the part from the entropy of where the solute molecules are

Term 4 is the one that gives the effect we discussed above.Term 3 is negligibly small for dilute solutions.Term 2 doesn't change when a solvent molecule evaporates, because the number of solute molecules doesn't change.Term 1 is the same as for pure solvent.

So the free-energy change when a solvent molecule leaves to go to the vapor (or the solid) is the same as for the pure solvent plus the term we calculated which depends only on the solvent and the density of solute molecules.

This is the sort of argument which must seem boring to most people, but some of us get a big thrill when some exact rigorous result like that pops out of the murk of complications that are important for most problems.

As I said, things get a little more complicated if there are long-range electrostatic interactions, so an update will follow.

mike w, again

p.s. A preliminary calculation of what happens with salts (ions) indicate that they only have the same effect on the boiling point as other solutes when their concentration is significantly lower than the background ionic concentration in the solvent. In water that's 10-7 M each of H+ and OH-. So for the sorts of salt concentration which have a major effect, electrostatic interactions make the effect per ion different from the result per molecule for non-ionic solutes.

(published on 10/22/2007)

Follow-Up #3: Why doesn't particle size affect vapor pressure?

Q:

i wanted to know that why does vapor pressure not depend on size of particles?? and why doesnt le-chatelier's principle come into play in this situation??- priya (age 17)india

A:

I assume that here you mean that the size of solute particles doesn't affect the vapor pressure of the solvent. This rule is true for sufficiently dilute solutes. We've discussed this very same question before, although it might have been hard to find, so I've marked it as a follow-up.